In a third-generation mobile communication system, an adaptive radio link, such as adaptive modulation/demodulation, HARQ (Hybrid Automatic Repeat Request) and scheduling, is employed to increase transmission efficiency of data packets.
Controlling such the adaptive radio link is carried out using an individual or common control channel, and link parameters being used in a data channel, which is transmitted substantially simultaneously with the control channel, are reported to each user terminal.
For example, in case of an Adaptive Modulation and Coding (AMC) scheme, the control channel transmits a modulation scheme and a coding rate of the data channel. In case of HARQ, the control channel transmits information such as the packet number of a packet to be transmitted on the data channel and the number of retransmission times. In case of scheduling, the control channel transmits information such as a user ID.
According to HSDPA (High Speed Downlink Packet Access) standardized in 3GPP (Third Generation Partnership Protocol) for third-generation mobile communication systems, transmission of control information as shown in TABLE 1 is carried out by use of the common control channel called HS-SCCH (Shared Control Channel for HS-DSCH), as described in Non-patent document 1.
TABLE 1Channelization-code-set information7bitsModulation scheme information1bitTransport-block size information6bitsHybrid-ARQ process information3bitsRedundancy and constellation version3bitsNew data indicator1bitUe identity16bits
Further, according to the above-mentioned HSDPA, when the AMC scheme is applied in FIG. 1, illustrating the relationship between the radio environment and the transmission speed, under good propagation condition in the varied radio environment I (exceeding a threshold level TH), high-speed data transmission is carried out by setting the modulation scheme to be 16 QAM (Quadrature Amplitude Modulation) with an increased coding rate.
On the other hand, under bad propagation condition (below the threshold level TH), data transmission is carried out at low speed by setting the modulation scheme to be QPSK (Quadrature Phase Shift keying) with a decreased coding rate.
As such, by changing the user transmission speed using the AMC scheme, it is carried out to maintain the quality constant. Namely, as shown in the above FIG. 1, according to HSDPA, the modulation scheme and the coding rate of HS-DSCH, user data, are made variable according to the propagation condition I. Further, HS-SCCH, which is control information related to the above user data, is also transmitted together with the user data (HS-DSCH).
However, at this time, in regard to HS-SCCH, the control information, as shown in FIG. 2, illustrating the relationship between the radio environment and the information amount, the coding rate of error correction coding for control information IV is constant, and accordingly, irrespective of good or bad radio environment I, an information amount to be transmitted becomes constant.
In the above case, when the radio environment I is in good condition, quality becomes excessive to the control information for transmission.
Further, in a next-generation mobile communication system, in order to realize high-speed data transmission, multicarrier transmission and MIMO (Multi Input Multi Output) transmission using a plurality of antennas are employed. In this case, it is possible to further improve a transmission characteristic, using adaptive control of radio parameters on a subcarrier-by-subcarrier basis and on a basis of each transmission antenna.
When the above MIMO is employed, controlling whether MIMO is to be applied or not is carried out depending on good or bad propagation condition I, as shown in FIG. 3.
Namely, in FIG. 3 illustrating the relationship between the application or non-application of MIMO and the transmission speed, when MIMO is applied, transmission speed becomes high, whereas in the opposite case, the transmission speed becomes low.
Further, formerly, the applicants of the present invention have proposed an invention of selecting one control channel format from among a plurality of control channel formats, each having a different information amount, depending on a predetermined condition (whether MIMO is applied or not) in a transmission system employing MIMO, and transmitting the control channel using the above selected control channel format (International Application Publication WO/2006/070466: Hereafter simply referred to as prior application.)
The above prior application is targeted on the assumed cases that the number of control channel information bits differs depending on whether MIMO is applied or not. As a prerequisite, the number of information bits IV is increased when MIMO is applied (period III) for user data as shown in FIG. 4, while the number of information bits is decreased when MIMO is not applied for user data, as shown in FIG. 5.
Accordingly, as shown in FIG. 6 illustrating the relationship between the MIMO application and the control channel information amount, in the period III of the propagation environment I in which MIMO is applied, there is a problem that the number of variable parameters increases, and the number of information bits required for the control channel increases. Further, when the number of simultaneous multiplex users in a single frame increases, there is a problem that the control channel information also increases in proportion to the number of users.
[Non-patent document 1]
    3GPP TS 25.212 V5.9.0 (2004-06)